Void-containing resin films or sheets are used as, for example, illumination members of electronic devices, illumination members for household use, and inner lighting sign boards, by virtue of, for example, their heat-insulating properties, cushion properties, and light-transmitting properties (or light-blocking properties).
Particularly in recent years, as liquid crystal TVs and computers are more and more popular, such a reflective plate that has higher reflectivity is required to be used as a reflective plate of liquid crystal devices.
In one technique for providing a reflective plate with higher reflectivity, a large number of fine voids are formed in a polyester resin (see, for example, Patent Literatures 1 to 3). When a void-containing layer is formed as a result of formation of fine voids in a polyester resin, the reflectivity of the polyester resin increases by virtue of the void-containing layer.
The technique described in Patent Literature 1 involves addition of inorganic fine particles to a polyester resin film, so that voids are formed within a film as the inorganic fine particles are separated from the resin interface during the stretching of the resin into a film. According to the technique described in Patent Literature 1, the addition of inorganic particles attains void formation to increase the reflectivity of the film.
However, the technique described in Patent Literature 1 requires not only sophisticated techniques and devices for dispersing fine particles, but also additives to reduce the aggregation of particles and pre-treatments of the fine particles, resulting in a complicated production process and increased cost.
In addition, if a foam layer is formed in the proximity of the surface of the polyester resin film, the surface smoothness may be impaired due to foaming.
In the technique described in Patent Literature 2, a major resin component (such as polyester) and another resin component incompatible with the major resin are mixed/kneaded to form a two-phase structure (such as islet structure). The interface between the major resin component and the other resin component added to and kneaded with the major resin is cleaved as the resin is stretched into a film, thus resulting in the formation of voids. The size of the incompatible phase may be made uniform, so that the voids can be easily controlled and the performance of the reflective plate can be improved.
When the technique described in Patent Literature 2 is used to produce a film, generally, an islet structure is formed and then its interface is cleaved as the resin is stretched into a film, thus resulting in the formation of voids. When this mechanism is employed, it would be difficult to form islets small enough to provide a desired two-phase structure. As a result, the resulting voids may not be sufficiently small (i.e., difficult to control the size).
In addition, if a foam layer is formed in the proximity of the surface of the polyester resin film, the surface smoothness may be impaired due to foaming.
Each of the techniques described in Patent Literatures 1 and 2 involves mixing into a major component other components that serve as nuclei to form voids. These components remain within the voids and may hinder the increase in reflectivity. In addition, the systems used in these techniques include a resin and an inorganic material or include different resins, and are therefore difficult to recycle.
A technique described in Patent Literature 3 involves exposing a resin film to an inert gas under pressure to impregnate the resin film with the inert gas, and stretching the resin film under atmospheric pressure to form a porous stretched resin film. Since this technique uses a gas as a source of voids, the problems involving degradation of reflectivity and recyclability can be avoided.
However, to impregnate the film with the inert gas under pressure, the entire film needs to be treated under a high pressure of several tens atm or over one-hundred atm. This requires a large-scale facility and can significantly add to the cost as compared to typical film-making apparatuses that involve melting and stretching of the film. In addition, the large volume of inert gas used in this technique requires additional equipment and countermeasures to ensure safety of operators, further adding to the cost. This technique also requires sophisticated control of conditions of the production process for ensuring uniform foaming.    Patent Literature 1: Japanese Patent (JP-B) No. 3067557    Patent Literature 2: Japanese Patent Application Laid-Open (JP-A) No. 2005-281396    Patent Literature 3: JP-A No. 2006-8942